1. Field of the Invention
This invention relates to a process for preparing poly(paraphenylene terephthalamide) fibers dyeable with cationic dyes and the dyed fibers produced thereby.
2. Description of the Prior Art
Fibers of poly(paraphenylene terephthalamide) offer high performance features such as flame resistance, very high tenacity and modulus, strength retention at elevated temperatures and good flex life. Accordingly, fibers of poly(paraphenylene terephthalamide) have found utility in various applications such as protective clothing, flame-resistant draperies, upholstery and carpeting. Often it is desirable for such end-use applications that the poly(paraphenylene terephthalamide) fibers be dyed. It is known that the molecular features of high crystallinity, stiff molecular chain, high interchain bonding forces and the like which contribute to the superior properties of the poly(paraphenylene terephthalamide) fibers make dyeing of these fibers difficult.
Various techniques have been proposed for dyeing fibers of poly(paraphenylene terephthalamide). For example, the dye can be incorporated into the solution from which the poly(paraphenylene terephthalamide) fibers are spun, see for example U.S. Pat. No. 3,888,821 and British Patent No. 1,438,067. However, making fibers of poly(paraphenylene terephthalamide) requires the use of extreme spinning conditions which can degrade the dye. This "spun-in" route has other problems such as interfering with fiber formation and contaminating the equipment. Further, it often is not economical to prepare and store separate inventories of fibers for each color of interest.
British Patent No. 1,438,067 to Moulds and Vance teaches imbibing an impregnant into never-dried fibers by passing the never-dried fibers through an aqueous bath containing the impregnant prior to dyeing. The impregnant serves as a "structure prop" which prevents collapse of the water swollen fibers on drying. Preferably the impregnant is a material selected from the group consisting of antioxidants, UV screeners, dyes, antistats and flame retardants such as tetrakis(hydroxymethyl)phosphonium chloride or oxide. The dried impregnated fibers may subsequently be dyed in an aqueous dye bath while corresponding fibers dried without impregnant may be dyed only under much more vigorous conditions, including the use of dye carriers, such as acetophenone.
Another process to dye fibers of poly(paraphenylene terephthalamide) is suggested in Japanese Kokai Patent No. Sho 52(1977)-37882. Aromatic polyamide fibers are pretreated with a sulfuric acid solution followed by treatment in a bath which contains 5 weight percent or more of a water soluble compound whose melting point is 100.degree. C. or higher. The fibers can be dried and subsequently dyed.